Female white-footed mice (Peromyscus leucopus) differ from most laboratory rodents in that they are markedly and overtly aggressive towards strange conspecific males and females, as well as towards other species. They also are markedly territorial in the wild. Aggression is most marked during lactation, but often persists afterwards. We plan to study the hormonal correlates of aggression and its development in these mice. We will measure endogenous levels of prolactin, progesterone, testosterone, estradiol and corticosterone during the various reproductive states from immaturity through estrous cycles, pregnancy, lactation, and adult reproductive quiescence. These same hormones will be administered to intact and castrate females at various stages of development to further ascertain their role in the development and expression of aggression. Once these basic hormonal patterns have been determined, the effects of increased density and low social rank will be studied. These studies will include 'juvenile' females whose maturation has been inhibited by (1) the presence of mature females, and (2) increased density. Finally we will explore the effects of aggression by female P. leucopus on individuals and populations of a subordinate species (meadow voles, Microtus pennsylvanicus. We wish to learn if aggression by the former will result in behavioral and physiological reactions in the latter leading to restricted movement and inhibition of population growth. These studies should lead to an increased understanding of aggressive behavior per se. We also should learn how social aggression of one species affects another. This may lead to a better understanding of the behavioral regulation of population growth. Overt aggression above some minimal level clearly is non-adaptive in its social milieu, but the maladaptive aspect of aggressive behavior is poorly understood and its regulation in the female essentially unknown.